Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ALUMINUM PHOSPHATE COMPOUNDS, COMPOSITIONS,
MATERIALS AND RELATED METAL COATINGS
The United States government has certain rights to this invention
pursuant to Grant Nos. F49620-00-C-0022 and F49620-O1-C-0014 from
AFOSR (Air Force Office of Scientific Research) and subcontract Grant No.
DE-FG02-01ER83149, from DOE (Department of Energy) each to Applied
Thin Films, Inc.
FIELD OF INVENTION
The present invention relates to the development of a new amorphous
inorganic oxide material, which is microstructurally dense and useful in a
number of applications where it can be used in powder, bulk, fiber, and as a
thin film or a coating. This invention is also related to surface modification
of
metals and alloys via application of thin films for providing protection
against
wear or abrasion, corrosion, and oxidation, over a range of temperatures and
harsh environments and for providing suitable high emissivity, non-wetting,
and non-stick surfaces.
There are a number of prior art patents related to synthesis of aluminum
phosphate materials primarily for use as a catalyst support including
crystalline
and amorphous forms. Most synthetic methods comprise of using a sol-gel
technique with raw materials that include commonly available salts of
aluminum and a variety of phosphorous sources including phosphoric acid,
ammonium hydrogen phosphates, phosphorous acid, and others. Many of these
methods yield highly porous and crystalline forms and few thermally stable
amorphous compositions (I1S Pat. No. 4,289,863, Hill et al.; US Pat.
Nos. 5,698,758 and 5,552,361, both Rieser et al.; US Pat. No. 6,022,513,
Pecoraro et al. US Pat. No. 3,943,231, Wasel-Nielen et al.; US Pat.
No. 5,030,43 l, Glemza; US 5,292,701, Glemza et al.; US Pat No. 5,496,529
and US Pat. No. 5,707,442, both Fogel et al.). Two prior art patents do teach
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formation of amorphous aluminum phosphate compositions. However, the
materials derived are highly porous which are desired for catalytic
applications.
US Pat. No. 4,289,863, , teaches a new method for synthesizing amorphous Al-
rich A1P04 compositions which are more thermally stable than Al-poor
compositions which crystallize at much lower temperatures. US Pat.
No. 6,022,513, teaches a slightly modified method for making Al-rich
compositions which yields a microstructurally different form of amorphous
aluminophosphate material. However, both synthetic methods yield highly
porous materials with surface areas over 90 to 300 square meters per gram with
a macropore volume of at least 0. lccs per gram as shown in the Pecoraro
patent
(pores are said to be between 60nm to 1000nm in US Pat. No. 5,698,758).
Much of the utility of such prior art amorphous materials is related to
their use as thin films on metals &'alloys, glass, and ceramic substrates. To
facilitate this utility, a combination of additional attributes would be
advantageous including a stable and low-cost precursor solution and an
environmentally-friendly, cost-effective, and versatile coating process
providing good adhesion with aforementioned substrates. There is a growing
need for coatings on metal and alloy substrates to provide protection and to
perform other surface-related functions. Most of the prior art methods require
either special pretreatments or additional layers to improve adhesion
particularly with metal and alloy substrates. The use of primers, including
phosphating agents, is well known in the paint industry. Conversion coatings
are well known in the art as pretreatment techniques to provide corrosion
protection and promote adhesion with paints. In this process, metal or alloy
surfaces are treated with acids or other chemical agents containing phosphates
or chromates which react with metal components of substrates to form metal
phosphate or chromate. However, these procedures are environmentally toxic
and the protection is not adequate. Similar primer layers are used for
applying
adherent coatings on metals and alloys. However, this adds to the cost and
imposes additional constraints for matching material properties within the
multilayered coating systems. It would be highly desirable to develop a one-
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step coating process that achieves both good adhesion and provides a
substantially pore-free amorphous inorganic layer for corrosion protection and
other purposes.
Prior art teaches that phosphates are excellent primers to improve the
adhesion on metal substrates. Several patents are based on using phosphate as
functional groups for better adhesion to the metal surfaces. See for example
US Pat. No. 6,140,410. A phosphate monomer is selected to provide phosphate
reactive groups in the main chain of the ultimate phosphated polyurethane
resin
to improve adhesion of the polyurethane resin to a metal by the formation of P-
-O-/Ma~ ionic bonds. See for example US Pat. No. 6,221,955. Adhesion
between a metal and a polymeric material is enhanced by contacting the metal
surface with a non-phosphate adhesion-promoting composition prior to
bonding the polymeric material to the metal surface US Pat. No. 6,554,948.
Numerous studies have been conducted as regards the application of thin
layers on steel surfaces by means of sol gel techniques. For example,
stainless
steel surfaces have been coated with zirconium dioxide layers to improve
corrosion resistance. Borosilicate glass layers have also been studied.
However, it was found that the refractory systems (high melting oxides such as
ZrO~) do not result in dense layers via said techniques and that the
borosilicate
glass layers could only be applied in layer thicknesses of significantly below
1 micron so that sufficient mechanical and chemical protection could not be
secured. (Sol-gel coatings on metals by M. Guglielmi, Journal of sol-gel
science .and technology, 8, 443-449 (1997); Sol-gel methods for oxide coatings
by L.F. Francis, Materials and Manufacturing Processes 12, 963-1015, 1997).
It is desirable to use an amorphous dense coating that is thermally durable
and
stable to protect various substrates. The primary advantage of an amorphous
coating is that, if developed by a suitable process, it can provide a hermetic
seal
over a substrate such that access of gas or liquids that can potentially
corrode
the substrate is avoided. Many methods have been developed to deposit
uniform crystalline coatings that are substantially pore or crack-free.
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Crystalline coatings do not provide hermetic protection from gas or liquid
exposures.
Silica-based amorphous coatings have been developed and a recent
patent prescribes a unique way to deposit such coatings (US Pat.
No. 6,162,498). However, the coating is not durable under certain harsh
conditions and are not thermally stable at elevated temperatures or do not
serve
adequately as a transparent coating on glass due to processing limitations.
High temperature stable glassy or vitreous coatings have also been developed
by initially coating substrates with a slurry of glass frits and subsequently
treating the coated material to high enough temperatures to melt the glass
frits
and form the vitreous coating. Vitreous enamel coatings have been in
existence for many decades with many different compositions. However, they
are usually thick and are porous and deform at elevated temperatures.
Although hermetic protection may be achieved with this process, the
requirement of high temperature processing to melt the glass frits may degrade
the substrate and if low melting glass compositions are selected, they may not
be durable due to the presence of sodium.
US Pat. No. 6,403,164 discloses a method to use organic-inorganic
hybrid films to provide protection against corrosion and for other uses.
Although the deposited films are dense and pore-free, they are not suitable
for
high temperature applications (above 300°C) and are relatively soft due
to
presence of organic material in the films. Such films are not wear or abrasion-
resistant.
Prior a.rt coatings have also included amorphous aluminum phosphate on
metals derived from various methods. British Pat. No. 1,451,145 discloses a
method to form hydrated form of aluminum phosphate coatings on metals
using a chemical solution method. Due to the low temperature curing methods
and presence of water (hydrated form), such coatings are not hard and robust
enough to withstand abrasion encountered in many applications and are not
microstructurally dense in an inorganic form to provide adequate oxidation or
corrosion protection.
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British Pat. Nos. 1,322,722, 1,322,724, and 1,322,726, and published
article entitled " Novel, low curing temperature, glassy, inorganic coatings,
derived from soluble complexes of aluminum and other metal phosphates" ,
(Chemistry and Industry, vol. l, (1974) 457-459) disclose utilizing a soluble
polymer complex comprising of aluminum phosphate with HCl and hydroxyl-
organic ligand. Although dense amorphous aluminum phosphate films have
been reported utilizing this method, there are several shortcomings which
relate
to their poor performance and make it impractical for commercial use. First,
the films contain residual chlorine (minimum of one weight %) which is not
desirable for many metals and alloys. Second, as the film cures, toxic HCl gas
is released (complex contains one mole HCl for every mole of A1P04) which is
a significant environmental concern. Third, the synthetic process is
relatively
complex involving isolation of the complex in crystalline form and then
dissolving it in appropriate solvents making it difficult to implement in
practical applications.
Inert and/or vacuum treatments are necessary to produce the precursor in
the aforementioned prior art and, in addition, it is not clear whether the
prepared precursor solution has sufficient shelf stability, or if the solution
decomposes upon exposure to the ambient (a potential concern due to the
presence of volatile organics, such as ethanol, present as a ligand). No
specific
examples were given related to deposition of filins on metal substrates or
their
corresponding behavior in an oxidation or corrosion tests. Due to the highly
acidic nature of the precursor solution, metal or alloy substrates may be
subjected to significant corrosion from chloride attack during film
development. In addition, due to the lower curing temperature, adhesion to
substrates may not be sufficiently high to yield durable films. Although
curing
temperatures ranging from 200-SOOC were suggested, most often curing
temperatures below 2000 were used and no specific example of films cured at
SOOC was provided and no microstructural information was given. In addition,
the coatings were found to adhere to molten aluminum. However, aluminum
phosphate, in pure crystalline or amorphous forms, is chemically compatible
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with molten aluminum and has been found to be non-wetting due to low
surface energy. Based on the poor adhesion of the prior art coatings, it is
suspected that the coating is not chemically durable (due to presence of
chlorine or poor film coverage or poor high temperature properties) and that
the
surface energy is not sufficiently low such that its applicability for non-
stick or
non-wetting applications may not be exploited.
In the aforementioned prior art, in addition, silicon and boron additions
were needed to extend the amorphous nature of the material. Even with these
additions, sufficient crystalline content (tridymite and cristobalite) was
present
after annealing the powder materials to 1090°C for 3 hours. As
explained
below, for the present invention, substantial amounts of non-crystalline
content
with only the presence of tridymite phase were found for materials with
varying Al/P stoichiometry after heat treatment at much higher temperatures
and extended time periods. It is not uncommon that amorphous materials
produced using various techniques may have distinct structural or network
moieties such that their atom diffusivities and high temperature behavior may
vary significantly. It appears that the network structure of the material
derived
under the aforementioned patent does not pxovide for a robust microstructure
and may not be suitable for use especially at elevated temperatures.
Thus, the material produced in prior-art methods is not microstructurally
dense or robust enough to provide the desired protection. In addition, none of
the prior art methods provide a suitable process or precursor solution that is
economical, stable and clear, and can be applied using a variety of well-known
techniques such as dip, spray, brush, and flow. Furthermore, none of the
processes associated with prior art methods offer the ability to provide good
adhesion with substrates that is critically important for most applications.
The
prior art coatings are either not durable under certain atmospheric conditions
or
under certain harsh industrial or use environments where materials are
subjected to thermal treatments or exposed to corrosive environments. Prior
art
inorganic coatings are also not completely transparent for use on glass where
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transmission properties are affected or other substrates where aesthetic
property
of the substrate (metallic appearance) needs to be preserved.
As a related consideration in the art, high strength metals such as
zirconium and zirconium alloys, titanium and titanium alloys, alloyed steels
and others become brittle when exposed to elementary hydrogen. This
embrittlement is known to be associated with the penetration of hydrogen
atoms into the metal lattice and has been the subject of extensive research.
In
spite of the considerable efforts to understand and thus combat hydrogen
embrittlement, this phenomenon is still a major cause of failure of vital
equipment such as heat transfer piping in nuclear power plants made of
zirconium alloys, supersonic aircraft segments made of titanium alloys and
machine parts such as bolts and shafts made of alloyed steels.
Efforts have been made in the past to overcome this catastrophic
phenomenon by the use of coatings as diffusion barriers which has largely
failed due to the extremely high permeability of hydrogen through most coating
materials. Palladium coatings might be theoretically considered in view of its
excellent properties and reasonable surface hardness, but commercially
attractive processes for applying palladium films require high plating rates.
But
such plating rates often lead to undesirable film properties. In many such
processes the palladium film is found to be brittle and susceptible to
cracking.
In addition, the process is expensive and produces environmentally toxic
byproducts
Metals whose position in the voltage series means that they react with
water are susceptible to corrosion and require a protective coating that
prevents attack by water and/or oxygen. For this purpose, the prior art
includes
a very wide variety of processes which have not yielded good results.
Anodizing of aluminum (Eloxal process) is well known method to form
protective alumina films, but the process does not yield pin-hole free alumina
films and the associated electrolytic process limits its applicability and
also
produces environmentally toxic materials.
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As evident from the preceding, a microstructurally dense form of
amorphous aluminophosphate would be very useful for a number of
applications. The prior art materials, for example, will not provide adequate
protection to substrates from corrosion or oxidation at elevated temperatures.
Porosity is not desired in their use in fiber form for use as reinforcement
for
composites or in optical applications. For photonic or laser applications,
aluminum phosphate is desired as a glass host material. Erbium-doped
phosphate glasses are being developed for use as fiber or planar waveguide
amplifiers wherein a dense material is required to transmit light without any
loss to scattering.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1. Cross-sectional TEM micrograph showing a well-adherent,
thin, uniform, dense and hermetic film of an inventive compound, composition
and/or material on AUS 304 stainless steel.
Figure 2. A chemical structure of aluminophosphorus complexes
present in the precursor solution of the inventive compounds, compositions
and/or materials.
Figure 3. FTIR spectra of an inventive compound, composition and/or
powder material with Al/P = 1.75/1 heat-treated in air (A) 150 °C (B)
1100 °C
Figure 4. Plot showing comparison of specific weight change between
coated and uncoated nickel-based superalloy coupons exposed to 1100°C
in air
for 100 h with one-hour thermal cycles.
Figure 5. X-ray diffraction spectra of coated and uncoated gamma-
titanium aluminide alloy showing substantial oxide growth on the uncoated and
minimal oxide growth on the coated substrate.
Figure 6. SEM micrograph of cross-section of coated (left) and
uncoated (right) AUS304 stainless steel annealed to 1000°C, 10 hr.
Figure 7. Cross-section SEM micrograph of a thin film of an inventive
compound, composition and/or material showing compatibility with molten
sodium sulfate for 120 hrs at 900°C in air, demonstrating the utility
of this
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invention to protect metallic components from corrosion in sulfur-bearing
environments, such as coal-fired power plants.
Figure 8. Coated and uncoated Aluminum 2024 alloy after 115 h
exposure to salt fog using ASTM B-117 specifications.
Figure 9. Transmission spectra of an inventive compound, composition
and/or material-coated sapphire (upper two lines) and uncoated sapphire (lower
line).
Figure 10. Schematic illustration showing a coating of an inventive
compound, composition and/or material (3) with adhesion layer (2) formed and
promoted in situ while ,depositing the film on a metal or alloy substrate (1).
Figure 11. A) Powder x-ray diffraction pattern of calcined material
from example 2, showing crystalline aluminum phosphate and corundum form
of alumina. B) 31P NMR spectrum of solution from example 2, showing a
resonance peak at -1 ppm, corresponding to unreacted triethylphosphate.
Figure 12. A) 31P NMR spectrum of solution from example 3, showing
a resonance peak at -5.9 ppm, corresponding to aluminophosphate complex.
B) X-ray diffraction pattern of calcined material from example 3, showing
predominately amorphous aluminum phosphate.
Figure 13. A photograph showing the sample of example 9 after
900°C,
30 minutes anneal in air.
Figure 14. Machined graphite pieces coated with amorphous aluminum
phosphate (left) as-received (not heated) and uncoated (right). The samples
pictured on the left and right were heated to 800°C, 2 hours in air.
Fig 15. Powder x-ray diffraction pattern of calcined material from
example 10. The peaks near two-theta values of 20.5, 21.5 and 35 are from
aluminum phosphate nanocrysta.ls embedded in the amorphous phosphate
matrix material. The other peaks correspond to La~P4013. Cu Ira radiation
used.
Fig 16. Powder x-ray diffraction pattern of calcined material from
example 11. The peaks near two-theta values of 20.5, 21.5 and 35 are from
aluminum phosphate nanocrystals. The peak (labeled "*") near two-theta value
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of 30 is from tetragonal zirconia. Based on x-ray data, the size of the
tetragonal zirconia nanocrystals are estimated to be about 7nm after
1100°C,
O.Sh anneal, 26nm after 1200°C, SOh anneal, and 170 nm after
1400°C, lOh
anneal in air. Cu I~oc radiation used.
Fig 17. X-ray diffraction pattern of calcined material from example 12.
The peaks near two-theta values of 20.5, 21.5 and 35 are from aluminum
phosphate nanocrystals. The peaks (labeled "*") near 16 and 26 are from
mullite (A16S12O13). The size of the mullite nanocrystals is 100nm after
1200°C, SOh heat treatment and 170 nm after 1400°C, lOh heat
treatment. Cu
Koc radiation used.
Fig 18. X-ray diffraction pattern of calcined material from example 13.
The peaky near 20.5, 21.5 and 35 are from aluminum phosphate nanocrystals.
The peaks near 26 and 37 are from anatase titania nanocrystals. The titania
nanocrystals are approximately 7rim in diameter. Cu I~oc radiation used.
Figure 19. Transmission electron micrograph showing nanoinclusions
of glassy carbon in powders of a compound, composition and/or material of
this invention.
Figure 20. Photograph of half coated stainless steel coupon after
immersion in molten aluminum 0750°C), showing non-wetting character.
The
dashed line shows the line of immersion in the molten aluminum.
Figure 21. X-ray diffraction patterns of 1018 carbon steel heat-treated
to 500°C for 30 minutes. (A) Coated with an inventive aluminophosphate;
and
(B) uncoated. The (*) indicates the substrate; the (1) indicates Fe3~4, and
the
(+) indicates Fe~03.
Figure 22. Raman spectrum of a coating of an inventive
aluminophosphate material on stainless steel. The labled (*) peaks are from an
adhesion layer between the coating and the substrate.
Figure 23. Grazing angle FTIR spectrum of stainless steel coated with a
thin film of an inventive aluminophosphate compound, composition and/or
material after exposure to the ambient. The labeled (*) peaks indicate
organics
absorbed on the coating surface.
to
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OBJECTS OF THE INVENTION
In light of all the shortcomings of prior art discussed above, there is a
need for a stable and microstructurally dense form of aluminophosphate which
is chemically durable and thermally stable for use in a broad range of
applications. Accordingly, it is an object of this invention to provide an
amorphous aluminophosphate compound, composition and/or material for
protective, functional, and multifunctional substrate coatings. Thus, there is
a
need to develop a durable glassy coating that is dense, smooth, continuous,
hermetic or substantially pore-free, and transparent which can be deposited on
a variety of substrates with excellent adhesion and at low cost with a simple
environmentally friendly process. Most of the current and emerging
applications utilizing metal/alloy substrates will require coatings that are
multifunctional such that other properties along with corrosion protection can
be induced. For example, antibacterial coatings are desired to limit the
spread
of bacteria and diseases for metal substrates. It would be desirable to
develop a
coating that provides both corrosion and antibacterial protection. Thus, a
thermally stable and robust glassy coating material need be developed with an
associated precursor system that can be flexible to induce multifunctional
properties, and is practical for use in industry and comme"rcial applications,
that
also offer low cost, simplicity, and environmental compliance.
It will be understood by those skilled in the art that one or more aspects
of this invention can meet certaita objectives, while one or more other
aspects
can meet certain other objectives. Each objective may not apply equally, in
all
its respects, to every aspect of this invention. As such, the following
objects
can be viewed in the alternative with respect to any one aspect of this
invention.
It is a further object of the present invention to develop a (preferably
transparent) glassy coating system which provides effective corrosion
protection for a very wide variety of metallic substrates, preferably in
combination with abrasion resistance properties.
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In accordance with the invention it has been found that this object may
be achieved by depositing an alumino-phosphate coating on the metal. Owing
to the inorganic network, the resultant coatings also possess abrasion
resistance
properties, which may be strengthened further by incorporating nanoscale
particles. Nanoparticles encompass dimensions ranging from about 1 nm to
about 500 nm. Another effect of incorporating the nanosized particles is that
such coatings remain transparent. The present invention accordingly provides
a process for protecting a metallic substrate against corrosion by forming an
inorganic glassy oxide film.
According to the present invention it has now been found that by using
specific precursors, vitreous layers can be formed on metallic surfaces, which
layers may be dimensioned less than about 10 microns. Surprisingly it has also
been found that such layers can be converted into dense aluminum-phosphate
films (for example on stainless steel or steel surfaces). Such films are about
a
few nanometers to about a few microns in thickness and form a hermetically
sealing layer which prevents or drastically reduces, respectively, the access
of
oxygen to the metallic surface and secures an excellent protection against
corrosion even at elevated temperatures. Such layers are furthermore abrasion-
resistant. Furthermore such coatings are flexible, i.e., bending or folding
the
surface does not result in any cracks or other deterioration of the layers.
Another objective of the present invention is to develop a stable and
microstructurally dense form of aluminophosphate material for use in the
aforementioned applications. A fiu-ther objective of the invention is to
develop
a love-cost, simple, and versatile chemical-solution based method to develop
the amorphous material in the form of powder, coating, fiber, and bulk
materials.
A yet another objective of the invention is to prepare a suitable clear
precursor solution that yields high quality dense coatings of amorphous
aluminophosphate. A further objective is to develop suitable precursor
solutions such that other additives can be added to the solution such that new
amorphous aluminophosphate compositions can be made. The additives can be
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added in a chemical form such that the solution is clear or the additives can
be
added in colloidal or powder form to yield a slurry-based solution. In any of
the precursor forms used, a cured material obtained may be in the form of a
nanocomposite (nanoparticles, nanocrystals or crystals embedded or
encapsulated in the amorphous aluminophosphate matrix) or exist as
uniformly-dispersed dopants within the glass matrix. In any of these forms,
the
additives, either individually or in conjunction with the aluminophosphate
matrix can induce specific functionality useful for many applications. Such
"mixed" aluminophosphate compositions can be formed as a powder or a
coating or a fiber or as a bulk material. It is another object of the
invention to
develop films of the inventive compounds, compositions and/or materials with
inclusions within the amorphous matrix material for inducing various functions
including, but not limited to, optical, chemical, catalytic, physical,
mechanical,
and electrical properties. Such inclusions can be produced in-situ during the
synthetic process and they may include metals, non-metals, and compounds of
any combination of elements. One such example includes formation of carbon
as nano-sized inclusions for providing high emissivity and enhances
mechanical properties. High emissivity coatings that are durable at elevated
temperatures are desirable for a number of applications where thermal
protection is desired or such coatings provide energy savings through re-
radiating incident heat fluxes in furnaces, ducts, boilers, heat exchangers,
and
the like.
It is an object of the present invention to provide a material having as a
feature of its molecular structure, an O=P-O-Al-O-A1 bonding sequence (with
organic and other ligands as may be attached to P and Al) regardless of P/Al
ratio and any additional metal therein to enhance coating properties or to
create
nanocrystals that induce or enhance chemical, physical, optical, electrical,
mechanical, and thermal properties (nanocomposite coatings).
It is an object of the present invention to provide surface modification of
metals or alloys with a material coating to provide corrosion or oxidation
resistance and/or to induce non-stick properties over a range of temperatures
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and environments; proven effective with stainless steel, aluminum alloys,
nickel-based superalloys, Inconel, and other steel alloys.
It is an object of tile present invention to provide such a material to
develop coatings from about 0.05 micron to about 10 microns (preferably about
100 nm, more preferably about SOOnm, most preferably about 1 micron); the
coatings are dense, continuous, smooth, uniform, and transparent. The
inventive compounds, compositions and/or materials and/or related coatings
are hermetic; that is, without open porosity or pathway fluid or gaseous
ingression, and/or micro-structurally dense; that is, substantially non-porous
and/or approaching zero pore volume. It is yet another object of the invention
to develop thin films in the range of about SOnm-about 10 microns that are
transparent or opaque as desired for any application. It is yet another object
of
the invention to enable the use of these thin films for applications that
require
maintenance of strict design tolerances such that substrate geometry or
features
do not need to be modified to accommodate the thickness of the films
deposited for protection or for other purposes of surface modification. With
films of the inventive compounds, compositions and/or materials, as thin as
about one micron or less, being sufficiently effective, no substrate
modifications are necessary for most applications.
It is an object of the present invention to provide cured coatings using
furnace or heat or infrared lamp or UV radiation (preferably @ 8000, more
preferably @ 600C, and most preferably @ SOOC); UV radiation along with
heat may cure the coating @ 2500. It is a related object of the present
invention to provide a curing process for excellent adhesion of the coating
material.
It is an object of the present invention to provide coatings deposited
using a dip or spray or flow or brush painting process. It is a further object
of
the invention to develop a process that utilizes a clear precursor solution
that is
stable (does not hydrolyze or decompose when exposed to ambient) and should
enable versatile deposition processes including dip, spray, flow, and brush
methods.
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It is an object of the present invention to provide a material that
functions as a protective coating in the short term, however, over long
exposures at elevated temperatures, promotes the formation of a protective
oxide scale thereunder that enhances the protection of substrates in the long
term, as can be achieved via a low partial pressure of oxygen at the
metal/coating interface during early stages, such that only stable oxides are
formed underneath. For instance, stainless steel AUS304 where, in preference
to a porous iron or manganese oxide, a dense chromium-rich oxide is formed;
note that higher chrome steels are preferred for this reason, but with the
material of this in~rention, even low Cr-containing steels will remain
oxidation-
resistant due to the promotion of Cr-rich oxide scales; proof of this was
obtained with x-ray diffraction and elemental profiles across the scales. The
same phenomenon occurs with nickel-based superalloys where alumina scale is
preferentially formed during early stages of oxidation; an order of magnitude
difference in oxidation rate is realized due to the presence of these
coatings.
This also allows the use of less-expensive alloys to be selected for certain
applications and the alloy composition can be tailored to promote other
properties (and not oxidation resistance), such as fatigue resistance, thermal
conductivity, thermal expansion, electrical properties.
It is an obj ect of the present invention to provide for thermal barrier
coating (TBC) aplalications; growth of alumina scale between the bondcoat and
ceramic coating leads to premature failure of the TBCs. The present coatings
can be deposited on MCrAIY type bondcoats and then TBC deposited
(preferably by e-beam P'VVI~), whereby the growth of alumina scale during
service is limited vhich will prevent catastrophic failure of TBCs due to
spallation; proof of principle already demonstrated with deposition of the
inventive compounds, compositions and/or materials on bondcoat showing
reduced weight gain from oxidation above 1100C.
It is an obj ect of the present invention to provide material coatings
sufficiently smooth to impart a low-friction surface (friction coefficients
below
0.1 were measured on coatings on steel alloys). This allows for use of the
is
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material as a high temperature solid lubricant or as a wear resistant coating
over a range of temperatures and environments; in this case, the inventive
compounds, compositions and/or materials may serve as a multifunctional
protective coating (nanocrystals within the material coating can be added to
improve wear resistance or tailor thermal properties). It is yet another
object of
the invention to reduce the surface roughness of said substrates which is
desired for many applications. The smooth nature of the films of the inventive
compounds, compositions and/or materials deposited allows for planarization
of most substrates. This will help in enhancing the non-wetting or non-stick
nature of surfaces and also induces a low-friction surface with the added
benefit of a lower surface energy attributed to the stable oxide surface on a
metal/alloy substrate. It is a related object of the present invention to
provide,
due to the hermetic nature of the coating, protection of any metal/alloy from
atmospheric corrosion.
It is an object of the present invention to provide protection of metal and
alloy surfaces that contain defects which causes "accelerated corrosion".
Access to moisture at these locations results in such behavior; upon oxidation
at elevated temperatures, after a period of time, micron-sized defects on
surfaces are enlarged to pits that are over 100 microns wide which eventually
coalesce on the surface leading to "breakaway'" oxidation. The inventive
compounds, compositions and/or materials conformally cover these defects and
eliminates accelerated corrosion. Because of this problem, metals and alloys
are subjected to extensive surface preparation (which results in labor and
material costs and generates waste) which can be reduced or eliminated with
the use of such coatings.
It is an object of the present invention to provide, due to the non-stick
properties, material coatings on metals and alloys that can be used in a
number
of applications where moving parts are used. Metal shafts, etc. are moved back
and forth during service and if there is any debris that sticks to the
surface, the
motion is affected and eventually leads to failure of the part. The inventive
compounds, compositions and/or materials coatings will help in not allowing
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unwanted debris from sticking to these parts and its smooth nature should
improve the sliding characteristics; it may also serve to act as a dielectric
or
insulation coating for certain applications.
It is an object of the present invention to provide a non-stick and
protective material highly suitable for use in the petroleum industry where
corrosive and high temperature environments are experienced. Deposition of
coke in ethylene cracking tubes is a major problem and the decoking process is
expensive and is time-consuming. The inventive compounds, compositions
and/or materials coatings can be deposited on top of alloy coatings to avoid
coke deposition.
It is an object of the present invention to provide protective coatings for
molten material processing; the amorphous, dense, and non-stick nature of the
present material is highly suitable for providing a non-stick surface. It can
be
used as a durable mold-release agent in die-casting. Most of the current mold-
release agents used in aluminum and other metal casting processes are
polymer-based which is durable only for one casting cycle. The durability and
hardness of inventive compounds, compositions and/or materials coatings will
help make it durable over several cycles which will save time and costs
(proven
to be an effective non-wetting protective coating for molten aluminum
processing with the present coated products lasting twice as long as other
coated products. The inventive compounds, compositions and/or materials can
also be deposited on top of enamel coatings to seal the highly porous
structure).
The present invention protects against other molten materials as well as
molten
aluminum, including molten polymers, molten glass and other non-ferrous
molten metals.
It is an object of the present invention to provide electrical insulation for
many metal and alloy parts used in a wide range of industries. In some cases,
both electrical insulation and corrosion resistance is required. The inventive
compounds, compositions and/or materials can serve as a suitable dielectric
for
a number of applications ; the pin-hole free nature of the coating is very
attractive for this purpose Dielectric coatings are desired for example on
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flexible solar cell metal substrates (next generation need). The stability of
metals and alloys used in plasma environments in the semiconductor or thin
film processing equipment is a major concern. Coatings can be deposited on
these metals and alloys to offer that protection. Additionally, it is an
object of
the invention to provide low dielectric constant films for semiconductors and
thermally stable low observable coatings for defense applications.
It is an object of the present invention to provide an anti-tarnishing
protective coating on metals and alloys. Polymer products are used to deposit
a
protective coating on these parts (door knobs, etc.), but they are not
durable.
The inventive compounds, compositions and/or materials are durable and can
be transparent such that appearance is not affected.
It is an object of the present invention to protect metallic alloys against
hydrogen embrittlement.
It is an object of the present invention to protect metals and alloys from
corrosion and oxidation under thermal cycling conditions while the coating
remains adherent to the substrate.
It is an objective of the invention that such coatings can be deposited on
substrates including, but not limited to, glass, metal, alloy, ceramic, and
polymers/plastics. It is a further objective of the invention to develop
coating
materials that are highly stable and possess low oxygen diffusivity such that
ultra-thin films of the material will provide adequate protection to
substrates.
This will be a significant advantage over prior art coating materials where
thick, non-hermetic coatings are used which crack or spall-off during thermal
cycling causing catastrophic failure of the part during use. This is
especially a
concern in aerospace and energy applications where extremely high
temperatures are used. It is yet another objective of the invention to allow
the
use of such coatings over a range of temperatures (cryogenic low temperatures
to above about 1400C) in a broad range of benign to harsh environments. It is
yet a further objective of the invention to utilize the low surface energy of
the
aluminophosphate material advantageously in applications where non-wetting
or non-stick proper=ties are desired. These may include, but not limited to,
non-
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wetting against water, solvents, chemicals, solids, molten salts, molten
metal,
and atmospheric contaminants (including organic matter).
It is an object of the invention to protect metal and alloy substrates in
both oxidizing and reducing environments. Metals and alloys are used in
variety of environments that include gases, liquids, and solids in contact
over a
range of temperatures. For examples, hydrogen or gases that induce a reducing
environment can react with metals and alloys to form undesirable reaction
products or hydrogen can diffuse into the material which causes the well-
known phenomenon of hydrogen embattlement. Fuel cells, for example,
operate in a combination of oxidizing and reducing environments and materials
used in their construction should be able to withstand the varying conditions.
An inventive compound, composition and/or material, used as a thin film, can
provide the necessary protection to various materials of construction used in
many of these applications requiring harsh environments, especially at
elevated
temperatures. Molten materials, including but not limited to, metal sulfates
(sodium sulfate, for example), metal vanadates, molten polymers (hot melt
adhesives), molten metal (aluminum, zinc), are used or are present in a broad
range of industrial processing environments that degrade metal/alloy
components during service. The inventive compounds, compositions and/or
materials, in thin filin form, can provide excellent protection due to its
thermal
stability and demonstrated durability with these corrosive materials. Due to
its
robust nature (low atom diffusivity), films as thin as 100 nm are sufficient
to
provide the desired protection. The ability to use such thin films are
particularly useful as they do not crack or spall upon thermal cycling. In
addition, they protect the underlying substrate from oxidation or corrosion
which fiu-ther helps in preventing delamination of the deposited film of the
inventive compounds, compositions and/or materials.
It is yet another objective of the invention to enable self absorption of
organic on the surface of the films of the inventive compounds, compositions
and/or materials deposited on substrates. Due to the presence of certain
organic contaminants in the atmosphere, surfaces of the inventive compounds,
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compositions andlor materials react with such organic materials, under ambient
conditions, forming a stable bond with the organic material or its modified
form via a self absorption process. Such organic films further lower the
surface energy of the composite structure, thus providing a hydrophobic or
non-wetting surface. Organic films can also be deposited over the film of the
inventive compounds, compositions andlor materials including, but not limited
to, oleic acid and organo-silanes, using simple dip-coating process. The
organic layer present is characterized by observation of an organic group on
the
surface using Fourier transform infra-red spectroscopy (absorption bands at
2994, 2935, 1702, 1396, 1337 and 972 cm 1 are observed which is attributed to
an organic group attached to the surface of the inventive compounds,
compositions and/or materials).
Organic layers can be deposited on the surface of the inventive
compounds, compositions and/or materials to promote hydrophilic behavior
such that bonding with certain materials are promoted. For example, adhesion
of polymers to metals and alloys is poor. The use of the surface of the
inventive compounds, compositions and/or materials, as an adhesive and
corrosion-resistant interlayer on metals to bond with various polymers and
ceramics will provide enhanced adhesion. Although the oxide nature of the
surface of the inventive compounds, compositions and/or materials itself can
promote direct adhesion with polymers, the adhesion characteristics can be
further enhanced by deposition of suitable hydrophilic organic layers on top
of
the film prior to bonding with polymers or other materials. Thus the surface
of
the inventive compounds, compositions and/or materials can be tailored with
organics to impart a hydrophobic or a hydrophilic character.
Other objects, features, benefits, and advantages of the present invention
will be apparent from the preceding, the summary of this invention, and the
following descriptions of various embodiments thereof, and will be readily
apparent to those skilled in the art having knowledge of various coatings,
protected substrates and/or composites. Such objects, features, benefits, and
advantages will be apparent form the above as taken into conjunction with the
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accompanying examples, data, figures and all reasonable inferences to be
drawn therefrom, alone or with consideration of the references incorporated
herein.
SUMMARY OF INVENTION
It was surprisingly found that a microstructurally dense amorphous
aluminophosphate material can be prepared using a low-cost precursor of
phosphorous pentoxide and hydrated aluminum nitrate, in ethanol or other fluid
media. Pyrolysis of the precursor at temperatures above SOOC yields a stable
microstructurally dense amorphous aluminophosphate material which is
resistant to crystallization up to 14000.
More importantly, it was surprisingly found that the precursor solution
has excellent film forming and adhesion characteristics to metal & alloy,
glass,
and ceramic substrates. Without being bound to any theory, it is proposed that
the adhesion is primarily promoted by phosphate bonding between the
constituents in the precursor solution and the metallic substrate. As
mentioned
above, phosphate bonding is well known for improving adhesion between
metal and inorganic and between ceramic materials. The higher curing
temperatures utilized in the present invention (above SOOC) helps in promoting
the adhesion. As the precursor is decomposed in ambient air at these elevated
temperatures, some oxidation of the metal substrate is induced which leads to
the formation of metal oxide (either as layer or as discrete islands). The
phosphorous contained in the precursor , at least partially, bonds with the
oxide
via a phosphate link, which enables good adhesion between the substrate and
the deposited film after curing. Prior art methods do not offer this
advantage.
This leads to a well-adhered film without requiring any special pretreatment
or
separate deposition of an underlayer to promote adhesion.
Embodiments of the aluminophosphate compounds, compositions
and/or materials of this invention inventive compounds, compositions andlor
materials are available under the Cerablak trademark from Applied Thin Filins,
Inc. Various considerations relating to this invention are disclosed in US
Pat.
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Nos. 6,036,762 and 6,461,415 and pending patent application nos. 10/266,832
and PCT/LJSO1/41790, each of which are incorporated herein in its entirety.
Post analysis of metal-coated films with the inventive compounds,
compositions and/or materials show characteristics of an "interfacial
layer°° that
is different in its chemical form compared to the substrate or the deposited
film.
Observing coated metals or alloys, under the optical microscope, often reveals
a colorful layer underneath the transparent film of the inventive compounds,
compositions and/or materials. For example, x-ray diffraction of films
deposited on mild steel show peaks corresponding to the formation of iron
oxide (Fea03). However, in other cases, the interface layer is indirectly
observed. The TEM micrograph of the film deposited on stainless steel does
not, for instance, reveal the presence of an interlayer, however, FTIR and
Kaman spectroscopic analysis show absorption corresponding to bonds that
cannot be assigned to either the inventive compounds, compositions and/or
materials or the substrate or any oxide that may have formed on the substrate.
It is believed that M-O-P bonds are formed at the interface during the curing
process that helps in achieving the excellent adhesion observed. Thus the
final
architecture of the coated material can be defined to contain component
between the substrate and the aluminophosphate an additional interface or
adhesive layer, which may comprise of a continuous phosphate-bonded metal
oxide or an oxide layer linked to phosphate groups of the film, or mixtures
thereof. Thus, the benefits of utilizing the said precursor system along with
a
suitable curing process yields a well-adherent glassy film.
Upon exposing the coated alloy to higher temperatures (above 800C), it
has been observed that the oxide formation underneath the deposited coating is
substantially reduced, and furthermore, the composition of the oxide scale is
substantially different from that observed for uncoated materials. Without
being bound to any theory, it is believed that the coating, due to its low
oxygen
diffusivity, establishes a lower partial pressure of oxygen at the coating
metal
interface, at a given temperature, which helps in formation of more stable
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oxides of alloy constituents. Further evidence of this phenomenon with
specific examples are provided herein.
Using a dip-coating process (described in detail below), a thin, dense,
smooth, hermetic, transparent, and continuous glassy coating is formed on
substrate surfaces. The precursor solution has low enough viscosity such that
a
uniform film can be deposited on complex-shaped substrates. Various
examples described below provide evidence for its formation, durability, and
stability under harsh exposure conditions. Specific monomeric or polymeric
species in precursor solution facilitate the formation of hermetic and
continuous film. During curing of sol-gel films, many events take place almost
simultaneously: solvent evaporation, gelation and stiffening leading to
shrinkage and densification of the film. If stiffening of the film occurs at
an
early stage, less relaxation will be allowed and films will be porous and/or
cracked. Not wishing to bound by any theory we believe the combination of
low hydrolysis-condensation rates and fast removal of organics is the key to
form a crack-free, hermetic coating using the precursor solution discussed in
the present invention.
The material tends to form over a wide range of aluminophosphate
compositions and stoichiometries such that a particular Al/P ratio can be
selected to suit the needs for a specific application. Al-rich compositions
are
more thermally stable in the amorphous form. Stoichiometric or P-rich
compositions also yield a dense material, but the thermal stability is
limited.
However, they may be useful in applications where the temperature limit do
not exceed 1000C. Stoichiometric aluminum phosphate refers to a compound
or composition having an aluminum/phosphorous ratio of about 1l1.
Most surprisingly, it was found that the material has very low oxygen
diffusivity such that it can serve as an excellent protective coating on
substrates
susceptible to high temperature oxidation. Because of this unique property, to
serve as a protective hermetic coating, it is sufficient to deposit an ultra-
thin
dense film of the material at a thickness of about 0.1 micron, more preferably
at
a thickness of about 0.5 microns, and most preferably a thickness of about 1
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micron. Such thin coatings are not prone to cracking and delamination due to
thermal expansion mismatch between coating and substrate.
The low-cost of the precursor material and deposition process also
allows for its deposition as an overcoat or undercoat on conventional
coatings.
for example, it is well known in the art that thick (few mils) metal alloy
coatings (such as MCrAIY) are deposited on substrates used in turbine engines,
ethylene cracking furnaces, and the like. Deposition of an ultrathin overcoat
of
The inventive compounds, compositions andlor materials will provide a life
enhancement of the underlying coating and thereby an enhancement of the
substrate at very little additional cost. As an undercoat in MCrAIY with a TBC
on top to help form stable alumina thermally grown oxide films during service.
In addition, the inventive compounds, compositions andlor materials can be
applied in the field during a plant shutdown or during routine maintenance to
provide additional protection. ~larious examples are provided below that
demonstrates its ability to protect metals, alloys, and.ceramics from
corrosion
and oxidation at elevated temperatures.
Accordingly, from a broader perspective, the present invention includes
a composite comprising a metallic substrate, a substantially amorphous,
substantially non-porous aluminophosphate filin and a component
therebetween. Such a component comprises a phosphate group in bonded
interaction with an oxide of a metal component of the substrate. The
aluminophosphate film comprises an aluminum content about, less than, or
greater than stoichiometric on a molar basis relative to the phosphorous
content
of the film.
In certain embodiments, the film of such a composite fiuther comprises
nanoparticles, such particles including but not limited to carbon, a metal
compound and combinations thereof. Without limitation, metal compound
nanoparticles include those described herein, but can also be selected from
those materials described in the aforementioned incorporated patents and
patent
applications. Regardless, in certain embodiments, the substrate can be a steel
alloy such that the aforementioned phosphate group is in bonded interaction
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with an iron oxide, a chromium oxide or a combination thereof, and such
bonding interaction promoted, in situ, during curing in formation of the film
or
coating. Likewise, regardless of nanoparticulate inclusion or substrate
identity,
the aluminophosphate film of such a composite can have a thickness dimension
of about 0.05 micron to about 10 microns. In various embodiments, such a
film can be dimensioned from about 0.1 micron to about 1.0 microns. As
described elsewhere herein, depending upon thickness, such a film can be
transparent or opaque, as may be needed for a desired end-use application.
In part, the present invention can also provide a high-temperature stable
composition comprising an aluminophosphate compound, substantially
amorphous with carbon nanoparticles therein. As mentioned above, the
aluminophosphate compound of such a composition can vary over a range of
stoichiometric relationships. In certain embodiments, where compositional
stability may be required at temperatures up to and exceeding about
1400°C,
the aluminophosphate compound has an aluminum content greater than
stoichiometric on a molar basis relative to the phosphorous content.
Regardless, such a composition can further, optionally, include nanoparticles
of
a metal compound, as described above. Alternatively, as compared to the prior
art, such a substantially amorphous aluminophosphate compound is without or
substantially absent chloride ion, such absence as can be indicated with
chloride levels or concentrations less than those disclosed in the
corresponding
patents of the prior art.
In part, the present invention can also include a method of using an
aluminophosphate compound to lower the surface energy of the substrate.
Such a method comprises (1) providing a precursor to an aluminophosphate
compound, such a precursor further comprising an aluminum salt and
phosphorous pentoxide in a fluid medium; (2) applying such a medium to a
substrate; and (3) heating the applied medium for a time and at a temperature
sufficient to provide a non-wetting, substantially amorphous and substantially
non-porous a.luminophosphate compound on the substrate. In certain
embodiments, as disclosed herein and by way of those incorporated patents and
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applications, the fluid medium can comprise an alcoholic solution of an
aluminum salt and phosphorous pentoxide. Application techniques vary as
described herein, but include, without limitation, dip-coating and spraying.
Illustrating implementation of such a method is the use of an aluminophosphate
compound on a substrate for non-wetting interaction with molten aluminum.
Accordingly, the present invention can also include a composite
comprising a metallic substrate and a substantially amorphous, substantially
non-porous aluminophosphate film on the substrate, such that the composite
has a surface energy lower than that initially available through use of such a
substrate, alone, such a surface energy as would be understood by those
skilled
in the art and in accordance with the structural, compositional and/or
physical
relationships described herein.
In general, many polymer or organic materials are known to have the
lowest surface energy due to the terminating hydrocarbon groups with fluorine-
based compounds providing a more enhanced effect in lowering surface
energies. Such surfaces provide non-stick non-wetting or hydrophobic
property to such surfaces. Polytetra fluoro ethylene (PTFE) is the most well
known non-stick material widely used in many applications including
cookware. A surface energy value of about 18 mN/m2 has been measured on
PTFE surfaces. However, metal, ceramic, and glass surfaces have relatively
higher surface energies with metals and alloys, in general, exhibiting the
highest surface energies and glass having the lowest among these groups of
materials. For many applications, it is desirable to lower surface energies of
metals, ceramics, and glass. Surface modification techniques, including
deposition of polymers are routinely used in industry to provide a lower
surface
energy. Such properties can enable enhanced flow characteristics of fluids
(including molten polymers, oils, aqueous and other organic solutions),
maintain relatively clean surfaces, and provide a low-friction surface.
Polymer coatings are not durable. The Inventive Material deposited as a
relatively smooth, substantially non-porous, and amorphous film on steel
components yields a low energy surface (~32 mJ/m~). This is relatively close
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to surface energies of certain polymers such as polypropylene. With the hard,
thermally stable, durable, abrasion- and corrosion- resistant nature of the
film,
the low surface energy of the Inventive Material can be exploited for a number
of applications, including high temperature applications, such as protective
films for molten metal processing. In addition, the ability of the film to
reduce
surface roughness of substrates will further enhance performance during
service. It is also possible to lower the surface energy even further by
varying
the AI/P composition and other processing parameters.
For the purposes of the present compounds, compositions, materials
and/or methods, the following expressions) and word(s), unless otherwise
indicated, will be understood as having the meanings ascribed thereto by those
skilled in the art or as otherwise indicated with respect thereto:
"Aluminophosphate" means a compound, composition andlor material
comprising aluminum and phosphate. Without limitation, such a compound,
composition and/or material can be represented with a formula A1P04, wherein
the aluminum and phosphate components thereof can vary over the range of
stoichiometric relationships known to those skilled in the art made aware of
this invention.
"Corrosion" means to any change in the metal which leads to oxidation
(conversion) to the corresponding metal cation with formation of a species X.
Such species X are generally (optionally hydrated) metal oxide s, carbonates,
sulphites, sulphates or else sulphides (for example, in the case of the action
of
H2S on Ag).
"Metallic substrate" means any substrate which consists entirely of one
or more metals or has at least one metallic layer on its surface.
"Metal" and "metallic" mean not only pure metals but also mixtures of
metals and metal alloys, these metals and metal alloys as may be susceptible
to
corrosion, but for employment in conjunction with this invention.
"On" means, in conjunction with a compound, composition and/or
material coating of this invention the position or placement of such a
compound, composition and/or material coating in relation to a corresponding
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substrate, notwithstanding one or more layers, components, films and/or
coatings therebetween.
Accordingly, this invention may be applied with particular advantage to
metallic substrates comprising at least one metal from the group consisting of
iron, aluminium, magnesium, zinc, silver and copper, although the scope of
application of the present invention is not restricted to these metals. Among
the metal alloys which may particularly profit from the present invention,
mention may be made in particular of steel, titanium, nickel and copper
alloys.
Without limitation, specific fields of application and examples of the use
of the present invention include the following:
construction, e.g. support and shuttering material made of steel, face
supports, pit props, tunnel and shaft lining constructions, insulating
construction elements, composite sheets comprising two metal profile sheets
and an insulating metal layer, shutters, framework constructions, roof
structures, fittings and supply conduits, steel protection boards, street
lighting
and street signage, sliding and rolling lattice gratings, gates, doors,
windows
and their frames and panels, gate seals or door seals made of steel or
aluminium, fire doors, tanks, collecting vessels, drums, vats and similar
containers made of iron, steel or aluminium, heating boilers, radiators, steam
boilers, turbine parts, halls with and without internals, buildings, garages,
garden houses, facings made of sheet steel or aluminium, profiles for facings,
window frames, facing elements, zinc roofs;
vehicles, e.g. body parts of cars, lorries and trucks made of magnesium,
road vehicles comprising and including aluminium, electrical articles, rims,
wheels made of aluminium (including chrome-plated) or magnesium, engines,
drive elements for road vehicles, especially shafts and bearing shells,
impregnation of porous die cast components, aircraft, marine screw propellers,
boats, nameplates and identification plates;
household and office articles, e.g. furniture made of steel, aluminium,
nickel-silver or copper, shelving units, sanitary installations, kitchen
equipment, lighting elements (lamps or lights), solar installations, locks,
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fittings, door and window handles, cookware, fryware and bakeware,
letterboxes and box-like constructions, reinforced cabinets, strongboxes,
sorting, filing and file-card boxes, pen trays, stamp holders, front plates,
screens, identification plates, scales;
articles of everyday use, e.g. tobacco tins, cigarette cases, compacts,
lipstick cases, weapons, e.g. knives and guns, handles and blades for knives
or
shears and scissor blades, tools, e.g. spanners, pliers and screwdrivers,
screws,
nails, metal mesh, springs, chains, iron or steel wool and scourers, buckles,
rivets, cutting products, e.g. shavers, razors and razor blades, spectacle
frames
made of magnesium, cutlery, spades, shovels, hoes, axes, cleavers, musical
instruments, clock and watch hands, jewelry and rings, tweezers, clips, hooks,
eyes, grinding balls, bins and drainage grilles, hose and pipe clips, sports
equipment, e.g. screw-in studs and goal frames.
Articles including metal parts like catalytic converters, photovoltaic
cells may be coated with the inventive material. Alternatively, high
emissivity
coatings may be useful for protective coatings for metallic thermal protection
systems, as well as increasing heat transfer efficiency for industrial and
consumer use, such as glass manufacturing, energy and metal manufacturing,
as well as duct linings, firewall materials, heat shields for xenon lights,
and
high temperature filters for liquid non-ferrous metals.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE
INVENTION
The inventive compounds, compositions and/or materials is a sol-gel
derived amorphous aluminum phosphate-based material. The inventive
compounds, compositions and/or materials can be synthesized over a wide
range of aluminum to phosphorous ratios, including from about 1/1 to shout
10/1. The inventive compounds, compositions and/or materials is highly inert
to chemical attack, thermally stable beyond 1400°C, and is sufficiently
transmissive to light in the visible, IR, and UV ranges (200-2500nm). High
temperature oxidation tests have shown that the inventive compounds,
compositions and/or materials is also highly impervious to oxygen ingress.
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The inventive compounds, compositions and/or materials can be
deposited as a dense, pinhole-free thin coating on substrates using a simple
dip,
paint, spray, flow or spin coating process at relatively low temperatures
(500°C
or above) (Figure 1). It has excellent potential to be scaled up without
significant capital investment to produce continuous coatings on a variety of
substrates. As a highly covalent inorganic oxide, the inventive compounds,
compositions and/or materials is chemically inert (like alumina) and thermally
stable material. The inventive compounds, compositions andlor materials is a
unique metastable amorphous material stable to temperatures beyond
1200°C.
Testing of The inventive compounds, compositions and/or materials has
demonstrated the electrical insulating property of the film and the
continuity,
hermiticity and, protective nature of the coating.
The species present in a precursor solution of inventive compounds,
compositions and/or materials can be used to derive the properties of the
solid
inventive compounds, compositions and/or materials. Based on the collective
experimental evidence, we believe the principal ingredients of the precursor
solution comprise of complexes that contain Al-O-Al linkages. This inference
is primarily based on identification of Al-O-A1 linleages in precursor
solutions,
dried gels, and calcined powders. 31P nuclear magnetic resonance (NMR)
spectra of the precursor solutions show at least one of two prominent peaks
near -5 ppm and -12 ppm, which is assigned to aluminophosphate complexes
(1) and (2) respectively, with a mixture of alcohol and water molecules
coordinated to aluminum (Figure 2). Further 31PNMR analysis of the precursor
solution shows predominantly the presence of two phosphate esters bonded to
one or two aluminum atoms. The reactivity of these complexes are sterically
restricted by the P=O groups and hydrolytically stable P-OR groups (See for
reference, Sol-gel synthesis of phosphates, J. Livage et al., Journal of Non-
Crystalline Solids, 147&148, 18-23 (1992)). Not bound by any theory, the
stability of the complexes can restrict the condensation of these complexes
(decreases the kinetics of condensation) forming an extended three dimensional
Al-O-P network. Accordingly, the shelf lives of precursor solution are
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extended and the solutions remain clear for several months to years. Further,
the alcohol-based solvent offers excellent film forming ability while the base
phosphate chemistry allows for chemical bonding with most substrates
resulting in strong adhesion.
These results support the formation of mufti-cation clusters with Al/P
ratio >_ 2 in solution leading to [O==P-O-Al-O-Al] cluster formation. Thus
both requirements of a) P=O and b) Al-O-A1 to be part of a cluster unit seem
to
be important. This trend is consistently observed with a number of other
synthetic routes for producing the inventive compounds, compositions and/or
materials. The species common to all solutions that yield inventive compounds,
compositions andlor materials are those consisting of at least [O==P-O-Al-O-
Al] links. Figure 3 shows FTIR of dried powder at 150 °C and calcined
at 1200
°C products, respectively. It is clear from FTIR data that at 150
°C, both P=O
and Al-O-Al species are observed. The observation of the P=O stretching at a
much higher frequency (1380 cm 1) indicates that the terminal oxygen atom in
P=O bond is uncoordinated.
Studying the evolution of the inventive compounds, compositions andlor
materials from the gel state also provides interesting insights. Upon
pyrolysis,
cross-linking of [O==P-O-Al-O-Al] moieties continue over a range of
temperatures eventually resulting in a "[-P04-A104-A106-A104-P04-]"
fragment in the high-temperature amorphous framework. The presence of this
type of linkage in the calcined material is established from combined data of
NMR and FTIR spectroscopy. The inventive compounds, compositions and/or
materials contains tetrahedral coordination for aluminum, along with
"distorted" octahedral aluminum, the intensity of which increases with excess
aluminum content. This is unlike the exclusive tetrahedral coordination for
aluminum observed in all crystalline polymorphs of A1P04. The ~~A1 NMR
data suggests a distorted environment for the tetrahedral Al, whereas the
corresponding 31P NMR shows an undistorted environment for [P04] groups.
Combining these two data we conclude that [P04] groups are linked only to
[A104] groups which in turn are linked to [A106] groups. Correspondingly,
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Al-O-Al bending mode vibrations at 825 clli lin the FTIR spectra, the
intensity
of which also scales proportionally with excess aluminum content, suggests a
direct linkage between [A106] and [A104] polyhedra.
The multicluster P-O-A1 complexes identified above represent a new
way for synthesis of amorphous oxide materials. Besides the precursor system
used in this specific case (aluminum nitrate and phosphorous pentoxide in
alcohol), essentially any precursor system that yields complexes with P=0 and
Al-O-Al moieties (which are linked with each other) will yield the inventive
compounds, compositions and/or materials. Regardless of the precursor system
used, the formation of these complexes appear to yield the inventive
compounds, compositions and/or materials. Such complexes may be further
modified with other additions (silicon, zirconium, lanthanum, titanium) which
can potentially enhance the amorphous characteristics or enhance the thermal
stability of these materials.
Although many coating techniques can be used with the precursor
solution, dip-coating, spraying painting and flow coating are most often used.
All are low-cost, easy to apply and scale up. We have been using these
techniques successfully on various substrates, including metals, alloys,
glass,
ceramics and others. The inventive compounds, compositions andlor materials
solutions show good wetting properties and is particularly significant when
alcohol (preferably ethanol, but other alcohols including, but not limited to,
methanol, isopropanol, butanol can be used as well) is used as the solvent,
although good wetting properties can be attained even using aqueous solutions.
Many oxidation studies have proven the hermiticity of the coating and the
advantage of thin inventive compounds, compositions and/or materials films.
Coatings on stainless steel coupons can withstand treatments of
1000°C or
more without cracking.
The coating composition employed according to the present invention
may be applied onto the metallic surface according to conventional coating
methods. Examples of techniques which may be employed are dipping,
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spinning, spraying or brushing. Particularly preferred are dipping and
spraying
processes.
The inventive compounds, compositions and/or materials solution has
been applied with a variety of methods and compositions. The inventive
compounds, compositions andlor materials has been coated onto a wide variety
of substrates, including stainless and mild steel, titanium, nickel, iron,
aluminum alloys glass, ceramics and carbon among many other substrates.
After application of the coating, it is dried to remove solvent and cured to
remove the organics and nitrates (or other salt components from the
precursor).
The coating can be cured in the furnace or with a portable quartz infrared
heat
lamp. The coatings cure quickly and are stable. Although the (final)
temperature of the thermal densification must also be determined in
consideration of the heat resistance of the metallic surface, said temperature
is
usually at least 300° C., particularly at least 400° C. and
particularly preferred
at least 500° C. If the metallic surface is sensitive to oxidation,
especially at
such high temperatures, it is recommended to carry out said thermal
densification in an oxygen-free atmosphere, e.g. under nitrogen or argon.
During the curing process, bonding with substrate materials is promoted.
IVIetallic or alloy substrates, in particular, partially oxidize and form an
oxide
scale (either partially or a continuous scale depending on substrate
composition, chemistry, and surface roughness) which then forms a strong
bond with the coating material during the curing process. In many cases, the
precursor solution may enable direct phosphate bonding of the metal surfaces
which also helps in improving adhesion. Although the exact nature of the
chemistry of the interface layer (existing between the substrate and the
applied
coating) is not known, evidence from optical microscopy, FTIR and Raman
spectroscopy, and x-ray diffraction methods show that the characteristics of
the
interface layer is different from either the substrate or the metal substrate.
This
has been observed both with mild steel and stainless steel substrates as
described in Examples 30 and 31.
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Thus the use of curing temperatures above SOOC in oxidizing
environment or ambient air is favorable for obtaining fully cured coatings as
well as attaining good adhesion with substrates. Although lower curing
temperatures can be used to cure the coatings by exposing for longer periods
of
time, temperatures in excess of SOOC is preferred to induce partial oxidation
of
the substrate or to promote direct bonding via phosphate groups with substrate
constituents. Those skilled in the art will recognize that the temperatures,
environments, and time of exposure can be adjusted over a wide range to
accomplish the various objectives discussed above. Use of higher temperatures
and higher partial pressure of oxygen in the ambient is preferred for fast
curing
suitable for many applications which will also reduce processing cost.
The adhesion with substrates can be further improved with altering the
Al/P ratio according to the substrate composition and oxide scale chemistry.
Excellent adhesion have been achieved with a number of alloy substrates
including various grades of steel alloys, nickel, Inconel, advanced nickel and
titanium alloys, aluminum, copper, titanium, and alloys thereof. Annealing the
coated materials to even higher temperatures provide further improvements in
adhesion, however, extensive oxidation of the substrate may lead to thick
enough oxide scales which may result in cracking or spallation at the oxide
scale/substrate interfaces. Annealing to ultra-high temperatures (above 1000C)
may result in some loss of phosphorous, depending on the environment,
however, the dense nature of the coating is still maintained such that
protection
is still considered good. Further description of the oxidation mechanism on
steel and advanced alloys are provided below with specific examples attached.
Aluminum and its alloys, due to their heat sensitivity can also be cured with
other surface heating techniques such as laser heating, IR lamps, and the
like.
Thus, if the heating is restricted to the surface of the substrate, mechanical
and
chemical degradation of the substrate can be avoided to a large extent since
compositional or microstructural changes occurring within the substrate may
affect its physical and mechanical properties. However, even curing of coating
on aluminum alloys have been accomplished using a fiirnace at temperatures
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ranging from 500-550°C. Such coated alloys have shown good performance
in
salt spray tests demonstrating good corrosion protection. For many substrates,
the application of a coating of an inventive compound, composition and/or
material can be combined with the tempering process, typically used to harden
metals and alloys. This will help in reducing the number of processing steps
required to make the final article for a given application.
The coating composition applied on the metallic surface will
subsequently be thermally densified to form a vitreous layer. Prior to said
thermal densification a conventional drying operation of the coating
composition at room temperature and/or slightly elevated temperature will
usually be carried out. It remains to be noted that the thermal densification
may optionally also be effected by IR, UV or laser heat sources. Also, it is
possible to produce structured coatings by selective action of heat thereon.
Slurries have also been made by dispersing a powder in a solution of an
inventive compound, composition andJor material. Slurry coatings were made
to increase the thickness or functionality of the coating. Different powders
were mixed into the solution. Slurry coatings can be applied by any of the
above coating methods. When synthesized as a powder, the inventive
compounds, compositions and/or materials contain nanoinclusions of glassy
carbon completely embedded in the amorphous material. These carbon
inclusions help to provide high emissivity characteristics to the powder. High
emissivity coatings can be made by making a coating from a slurry of black
compounds, compositions and/or material particles of this invention dispersed
in solution or a suitable medium. The inventive compounds, compositions
and/or materials may also be used as a protective binder for pigments. It is
also
possible to synthesize such compounds, compositions and/or materials without
carbon inclusions with appropriate selection of precursor formulations.
The low-cost associated with the present invention and coating
technology allows for combined options to be considered. It is expected that
the inventive compounds, compositions and/or materials can enhance the
oxidation resistance behavior of a wide range of alloys, even alloys with high
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oxidation resistance. It can be deposited on weld areas where morphological
non-uniformities and compositional variations are bound to exist. The
inventive compounds, compositions and/or materials are also amenable to field
repair, or could be applied during shut-downs where deposits on wash walls or
other areas are cleaned. Spraying is a suitable process for depositing
inventive
compounds, compositions and/or materials coatings, and could be used as a
field-repair process.
Examination of coated substrates under 1000x magnification, using an
optical microscope shows the continuous character of the coating. Compliance
of coatings of the inventive compounds, compositions and/or materials on steel
foil has been demonstrated where the foil has been bent (>120 degrees) several
times without any sign of delamination, thus demonstrating excellent adherence
of the thin film.
Thin coatings are often preferred to avoid delamination from thermal
treatments, for ease of application at lower costs and their compliance even
in
case of large CTE match between the substrate and the coating. If an
insulating
layer is required, thicker coatings are preferred for providing adequate
electrical insulation and for providing the desired diffusion barrier
characteristics during deposition of functional overlayers. Since the
inventive
compounds, compositions and/or materials are extremely inert to chemical
attack, and has a low dielectric constant, it should serve as an excellent
insulation and diffusion barrier around 500°C even at thickness of
about 2000-
SOOOA. A dielectric breakdown strength of 190V has been measured for the
inventive compounds, compositions and/or materials as a 100 - 500 nm thick
film on stainless steel. Such dielectric constants can range from 3.3 - 5.6.
The inventive compounds, compositions andlor materials significantly
limit the oxidation of metal/alloy substrates, and limit corrosion as well. In
addition to providing protection, the present invention appears to change the
chemistry of the growing oxide scale. Perhaps more significantly, the
invention can be used to mininuze the effects of accelerated corrosion due to
surface roughness, and eliminates corrosion pitting, which is often observed
on
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oxidized, metal/alloy substrates. Thus, a function of the inventive compounds,
compositions andlor materials coating is in the early stages of oxidation
where
it protects sharp edges, planarizes the surface, and defects in an alloy
surface.
The ability to protect nickel-based alloys at temperatures over
1000°C
for over 100h has been successfully demonstrated. Two grades of nickel-based
superalloys were coated with the inventive compounds, compositions and/or
materials (~l micron thick coating by dip coating process) and their oxidation
behavior was studied under thermal cycling conditions. Figure 4 show the
oxidation behavior of the coated and uncoated alloys. Each data point on the
curve represents a thermal cycle (RT-1100°C). Thus the cumulative
exposure
time at temperature was around 100 hours. Both coated materials performed
very well under the exposed conditions, especially considering the high
temperatures used.
The inventive compounds, compositions and/or materials coatings were
also tested on lnconel 718 at 760°C under thermal cycling conditions.
As
described before, the coated materials were tested for 20 thermal cycles where
subsequent surface examination showed the inventive compounds,
compositions and/or materials was effective in its protection and, more
importantly, no additional cracking or spallation was observed. The inventive
compounds, compositions and/or material were used to coat a y-titanium
aluminide alloy, and heat treated, with an uncoated alloy for 100h at
815°C.
The uncoated alloy showed extensive growth of ruble titania and corundum
alumina on the surface. The coated alloy showed significantly reduced oxide
growth, as seen by x-ray diffraction (Figure 5). All these studies combined
suggest the inventive compounds, compositions and/or materials have excellent
potential for use in protecting alloys intended for use a wide variety of
elevated
temperature applications.
The inventive compounds, compositions and/or materials coatings have
been shown to protect alloys against oxidation. Not wishing to be bound by
any theory, it is believed that during the very early stages of oxidation a
low
P~2 is established at the alloy such that only oxides stable in low P~2
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environments are formed (usually chromia for steels or alumina for nickel-
aluminum based alloys). To illustrate, a piece of type-304 stainless steel was
half coated with the inventive compounds, compositions andlor materials and
heat treated to 1000°C for 10 hours in air. The coated half shows a
dense,
uniform, chromia-rich scale, while the uncoated half shows a non-uniform
scale with deep pits of non-protective iron-rich oxide (Figure 6).
It appears that the inventive compounds, compositions and/or materials
provide significant benefits by modifying the growing oxide. Irrespective of
the selection of the alloy, issues related to roughness or surface defects are
bound to be of concern, and may require special pretreatments which can be
expensive, environmentally unfriendly, and will generate waste. Surface-
related defects affected the oxidation behavior for the uncoated materials.
Despite the high quality finish, these factors dominated the oxidation
behavior.
For the intended applications, alloy materials may be fabricated using a
wrought process which is certain to create surface defects, and this issue
must
be addressed. Furthermore, the simplicity and versatility of the inventive
compounds, compositions and/or materials coating process and the inexpensive
nature of the precursor solution may make it cheaper to deposit a compound,
composition andlor material coating of this invention rather than performing
many other, more expensive pretreatment of alloy surfaces.
For thermal barrier coating applications, growth of thick alumina scale
between the bondcoat and ceramic coating leads to premature failure of the
TBCs. Compounds, compositions andlor materials coatings of this invention
can be deposited on McrAIY type bondcoats and then TBC deposited
(preferably by e-beam PVD), whereby the growth of alumina scale during
service is limited which will prevent catastrophic failure of TBCs due to
spallation. A standard practice used in turbine manufacturing is to preoxidize
the bondcoat material (MCrAlY type compositions) to improve the adhesion
with the thermal barrier coating to be applied. Often, reducing environments
are used to promote the preferential formation of alumina scales (as opposed
to
spinel and other oxides), see for example US Pat. No. 5,856,027 Murphy. Such
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treatrnents in inert or vacuum significantly increases the cost of production
and
limits the production efficiency. A thin film deposited on MCrAIY can
promote the formarion of alumina scale even if it is annealed in ambient air.
In
addition, as the morphology of the surface is rough, sealing of defects by the
coating may provide an added benefit. Furthermore, the presence of the
oxidation-resistant glassy film may provide enhanced protection of the
substrate during subsequent use.
The inventive compounds, compositions and/or materials can protect
metals and alloys against corrosion from molten sulfates, encountered in
combustion applications. Its compatibility with the trisulfates vary
depencling
on the Al/P stoichiometry whereby Al-rich compositions appear to be more
compatible. The non-wetting character may be useful in limiting the adhesion
of ash particles to the metal/alloy components used in coal-fired combustion
systems and other power generation plants.
To assess the compatibility of the present compounds, compositions
and/or materials with sodium sulfate, coatings thereof were deposited onto
sapphire plates. Sapphire substrates were chosen to avoid the influence of
oxide scale on the compatibility test. Sodium sulfate was placed on the coated
sapphire pieces and annealed to 900°C, just above the melting point
(884°C).
Fig 7 shows a coated piece after 120 hrs of exposure. As apparent from the
micrograph, the inventive compounds, compositions and/or materials, even at
one micron thickness, were not degraded by the exposure.
Coatings for enhancing the corrosion resistance are suitable, for
example, for iron and steel products, especially profiles, strips, plates,
sheets,
coils, wires and pipes made of iron, of unalloyed, stainless or otherwise-
alloyed
steel, either bright, zinc-plated or otherwise-plated, semifinished forged
goods
made of unalloyed, stainless or other alloyed steel; aluminium, especially
foils,
thin strips, sheets, plates, diecastings, wrought aluminium, or pressed,
punched
or drawn parts; metallic coatings produced by casting or by electrolytic or
chemical processes; and metal surfaces enhanced by coating, glazing or anodic
oxidation.
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Coatings for enhancing the wear resistance are suitable, for example, for
jewelry, timepieces and parts thereof, and rings made of gold and platinum.
Diffusion barrier layers are suitable, for example, for lead fishing weights,
diffusion barriers on stainless steel to prevent heavy metal contamination,
water pipes, tools containing nickel or cobalt, or jewelry (anti-allergenic).
Surface leveling/frictional wear reducing coats are suitable, for example, for
seals, gaskets or guide rings.
Steel, ixon, aluminum and other alloys corrode readily in a humid and
salty environment. The salt fog test is highly accelerated, allowing useful
tests
in a reasonable amount of time. Coatings of the inventive compounds,
compositions and/or materials on aluminum and carbon steel coupons have
been tested for preliminary evaluation in the salt fog apparatus, and show
increased resistance to corrosion compared to aluminum coupons. After
testing, the uncoated coupons showed corrosion over the entire piece, while
most coated coupons only showed very localized corrosion with one of the
coated coupons showing virtually no sign of corrosion. The corrosion on the
coated pieces often occur where the surface finish is uneven. Fig 8 shows the
inventive compounds, compositions andlor materials coated and uncoated
coupons after the salt fog test.
Non-wetting behavior is helpful to prevent corrosion in a humid/rainy or
coastal environment. The inventive compounds, compositions and/or materials
show non-wetting behavior to water and other liquids. Non-wetting
characteristics are primarily due to the low surface energy associated with
the
high degree of covalency. It may also serve as an anti-static coating
preventing
solid particles, such as dust or lint from sticking to its surface.
Transmission to light is important for many applications. A glass
microscope slide coated with the inventive compounds, compositions and/or
materials was compared to an uncoated slide. Such compounds, compositions
andJor materials have been shown to be transmissive to radiation between 200-
2500nm. A coating was put on a sapphire plate, and the transmission
properties were compared to an uncoated sapphire piece, cut from the same
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large plate. Two coated pieces were tested, one with a thicker coating than
the
other. Fig 9 shows the transmission of the coated vs. uncoated sapphire
plates.
The inventive compounds, compositions and/or materials could be used
a protective coating against coke. Their non-stick properties, thermal
stability,
and protective nature is highly suitable for use in the petroleum industry
where
corrosive and high temperature environments are experience; deposition of
coke in ethylene cracking tubes is a major problem; decoking process is
expensive and is time-consuming; inventive compounds, compositions and/or
materials coatings can be deposited on top of alloy coatings to avoid coke
deposition. Coke formation is promoted by catalytic reaction between the
metal substrate and hydrocarbons present in the gas stream. A substantially
pore-free thin film, preferably hermetic, may serve as an excellent barrier to
prevent contact between the metal and hydrocarbons. In addition, carburization
of metal substrates degrade its mechanical properties. The coating will help
prevent carburization, of metals and alloys.
Yet another high temperature application of the inventive compounds,
compositions and/or materials relates to their use as a coating for high-
temperature protection of metal- or alloy-based thermal protection systems
(TPS) to be used in reusable launch vehicles (RLVs). Such a material provides
both oxidation protection to the underlying substrate and high emissivity
characteristics. The powders retain the black or dark color for over 100 hours
at 815°C and over 24 hours at 1100°C and retain high emissivity.
The present invention has also been demonstrated to protect substrates
from attack from molten non-ferrous metals, such as aluminum and zinc, as
well as molten polymers. The low surface energy of the inventive compounds,
compositions andlor materials allows it to remain non-wetting to these and
other materials.
The inventive compounds, compositions and/or materials can also be
used to provide a low friction surface. The friction coefficient of inventive
compounds, compositions andlor materials coatings on highly polished 440C
stainless steel substrates was measured to be around 0.1.
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The inventive compounds, compositions and/or materials show excellent
adhesion to metals, alloys and ceramic/glass substrates, upon heat treatment
to
form the inorganic material. When deposited on a metal or alloy, the heat
treatment allows a very thin oxide scale to form on the substrate surface,
which
enhances the adhesion of such compounds, compositions and/or materials to
the substrate.
Nanoparticles or nanocrystals of varying chemistries can be embedded
or encapsulated into the aluminophosphate amorphous material for inducing
various functions including, but not limited to, optical, chemical, catalytic,
physical, mechanical, and electrical properties. The prior art coatings are
not
robust enough to protect the nanocrystals and, in addition, processing of
nanocomposites with a porous host will be a significant challenge
EXAMPLES OF THE INVENTION
The following non-limiting examples and data, in conjunction with the
referenced figures, illustrate various aspects and features relating to the
compounds, compositions, materials and/or methods of the present invention,
including the preparation, application and/or use of corresponding films and
coatings on a variety of substrates, such compounds, compositions and/or
materials as are available through the synthetic methodologies described
herein. In comparison with the prior art, the present invention provides
results
and data which are surprising, unexpected and contrary thereto. While the
utility of this invention is illustrated through the use of several
aluminophosphate compounds, compositions andlor materials and composites
thereof, it will be understood by those skilled in the art that comparable
results
are obtainable with various other aluminophosphate compounds, compositions
andlor materials, as are commensurate with the scope of this invention.
Example 1
264 g of Al(N03)3-9H20 is dissolved in 300 mL ethanol. In a separate
container, 25 g P205 (or other soluble phosphate ester) is dissolved in 100 mL
ethanol which promotes the formation of phosphate esters and this solution is
then added to the aluminum-containing solution. This solution refluxed for
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time sufficient to promote the formation of complex esters containing Al-O-P
groups. This solution is clear and shelf stable for years.
Example 2
l9mL triethyl phosphate was mixed with 84 g of Al(N03)3~9HZQ in 181
mL of ethanol. After stirring for 30 min, the mixture was dried to form gel
powder and annealed to 1100°C for 1 hour in air. The x-ray diffraction
pattern
obtained shows highly crystalline aluminum phosphate and alumina phases,
indicating that this mixture does not form amorphous aluminum-rich aluminum
phosphate. The 31P NMR of the solution showed peaks near 1 ppm, indicating
that the phosphorous was not complexed with the aluminum to any significant
extent (Figure 11).
Example 3
The solution mixture of Example 2 was refluxed. After various periods,
some of the mixture was dried to a gel powder at 150°C for 1 hour and
then
annealed at 1100°C for 1 hour in air. As the refluxing time increased,
the
amount of amorphous aluminum phosphate increased. After 3.5 days of reflux,
a substantially amorphous phase is formed. The 31P NMR of the solution
showed peaks near -5 ppm, indicating that the aluminum was complexed with
the phosphorous (Figure 12).
Example 4
119.28 g Al(N03)3~9H~0 was dissolved in 510 mLl-butanol. In a
separate beaker, an appropriate molar amount of phosphorous pentoxide was
dissolved in 3lmL of 1-butanol. These two solutions were mixed together to
form a 1.75/1 Al/P 1-butanol based solution. The solution was dried to a gel
powder at 150°C for 1 hour and then annealed to 1100°C for 1
hour in air. A
jet black powder resulted (black color due to the presence of residual carbon
encapsulated in amorphous matrix), and the x-ray diffraction pattern indicated
that the powder was substantially amorphous. TEM analysis revealed that the
size of the carbon nanoinclusions was larger than those in powders derived
from the solution of Example 1.
Example 5
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Coupons of Inconel 718 were dip-coated with the solution of Example 1,
and cured with an IR lamp for 10 min. The coupons were heat treated
according to the schedule in Table 1 and examined under optical microscope
after 20 cycles. It was noticed that there was no additional cracking of the
coating compared to as-deposited coatings which showed some cracking near
the edges and few cracks in other areas. This demonstrates that the thin
nature
of the deposited film of the inventive material is not subject to cracking
from
thermal stresses even though the thermal expansion mismatch between the
substrate and the inventive material is significant.
Initial temperatureFinal temperatureTime to reach IW ration
(C) (C) final of hold
temperature (min)final temperature
20 468 35 50
468 100 75 0
100 538 35 30
538 100 75 0
100 607 40 30
607 100 85 0
100 760 52 1
760 20 95 0
Table 1. Thermal cycling profile from Example 5.
Example 6
A piece of stainless steel was dip-coated into the solution mixture of
Example 1. The coupon was dried with flowing air and heated with an IR lamp
for a sufficient time to cure the film (remove all of the organics and
nitrates) to
form substantially inorganic material. The resulting coating was uniform and
crack-free.
Example 7
A stainless steel coupon was coated using the solution mixture of
Example 1 by a roller. The roller was saturated with precursor solution and
rolled quickly and firmly across most of the stainless steel coupon. The
coupon
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is then heat treated with an IR lamp (as in Example 6) for sufficient time to
remove all organics and nitrates. The coupon is heat treated to 900°C
for 30
minutes in air, and the coated portion of the substrate remains shiny, while
the
uncoated section appears dull from formation of substantially greater oxide
scale (Fig 13).
Examt~le 8
2.2 grams of finely milled (sub-micron to few microns) amorphous
aluminum phosphate powder (black in color) derived using processes in the
aforementioned Examples was dispersed in 12 mL of ethanol and was added to
12 mL of the solution mixture from Example 1 to form a slurry for application
of coatings. This slurry solution was used to coat a piece of type 304
stainless
steel as in Example 7 that yielded a composite coating consisting of powder
dispersed in the amorphous matrix derived from the solution portion of the
slurry. The resulting coating had amorphous aluminum phosphate powder
evenly distributed in a crack-free amorphous aluminum phosphate coating.
The coating was substantially dark in appearance due to the presence of
particles in the said composite coating.
Example 9
As-machined graphite samples were coated with the inventive material
and annealed, along with an uncoated piece, to 800°C for 2 hours in
air. The
coated pieces retained their physical dimensions to a substantially greater
extent than their uncoated counterparts as seen in Figure 14 suggesting that
the
inventive material is effective in providing oxidation protection to graphite
and
carbon-based materials, including composites.
Example 10
19.47g PROS was dissolved in 200 mL ethanol. In another container
180.68 g Al(N03)3~9H20 was dissolved in 400mL ethanol. In a third container,
16.32 g La(N03)3~6H20 was dissolved in 100 mL ethanol.. All three solutions
were mixed together and stirred. A clear solution resulted. Some solution was
dried at 150°C in a convection oven and annealed to 1100°C for 1
hour.
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Crystals of La~P4013 and a predominately amorphous aluminum phosphate
were identified by x-ray diffraction (Figure 15).
Exam 1p a 11
Analogous to the preceding example, using similar methodology, a
zirconium-containing solution can be made. 6.46g Pa~5 was dissolved in 70
mL ethanol. In another container 59.9 g Al(N03)3~9H~~ was dissolved in 140
mL ethanol. In a third container, 1.49 g Zr0(N03)3~xH20 was dissolved in 10
mL of ethanol. All three solutions were mixed together and stirred. Some
solution was dried at 150°C in a convection oven and annealed to
1000°C for 1
hour. Crystals of tetragonal Zr02 and predominately arriorphous aluminum
phosphate were identified by x-ray diffraction (Fig 16). The relative amount
of
zirconia nanocrystals depends to some degree on the storage time of the
precursor.
Example 12
Analogous to the preceding example, using similar methodology, a
silicon-containing solution can be made 8.47 g P205 was dissolved in 90 mL
ethanol. In another beaker, 78.6 g Al(N03)3~9H20 was dissolved in 174 mL
ethanol. In a third container, 1.7g tetraethylorthosilane was dissolved in
lSmL
ethanol. All three solutions were mixed together. Some of the solution was
dried in a convection oven at 150C. Some of this dried powder was annealed at
1400°C for 10 hours. Crystals of mullite and predominately amorphous
aluminum phosphate were identified by x-ray diffraction (Fig 17).
Exam 1p a 13
Analogous to the preceding example, using similar methodology, a
titanium-containing solution can be made 0.2mL nitric acid was mixed with 9.8
4mL deionized water. 2mL titanium isopropoxide was added to this acidified
water solution, and a white precipitate resulted. This mixture was added to
86mL of the aluminum phosphate solution from example 1. This mixture was
dried at 150°C and annealed at 1000°C for 1 hour. X-ray
diffraction indicated
the presence of titanium oxide (anatase) and predominately amorphous
aluminum phosphate (Figure 18).
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Example 14
The solution of Example 1 is dried and heat treated to 1100°C for
1
hour. The powder is milled to 10-20 microns in diameter. The resulting
powder is jet black in color and the x-ray diffraction pattern confirms that
the
powder is substantially amorphous.
Example 15
The powder of Example 4 is examined in the TEM. Transmission
electron microscopy has shown nanoinclusions of glassy carbon embedded in
the amorphous matrix of the inventive material. These inclusions are about 2-4
nm by 10 - 40 nm in size (Figure 19). The inclusions are typical in appearance
of glassy carbon and EDS evidence has shown that these particles primarily
contain carbon .
Example 16
The powder of Example 14 is clispersed in the solution mixture of
Example 1. This slurry is painted on an alumina substrate. This coating is
dried in flowing air until the solvent has evaporated and then heat cured
above
500°C . The room temperature total hemispherical emissivity of the
coating, as
measured from 2 - 20 ~.tn, is 0.917.
Example 17
The coating of Example 16 is heat treated in air at 815°C for
100h. The
room temperature total hemispherical emissivity,of the coating, as measured
from 2 - 20 Win, is 0.908 suggesting that substantial portion of the
nanoinclusions of carbon did not oxidize and was well protected by the
substantially pore-free and dense matrix of the inventive material.
Example 18
The coating of Example 16 is heat treated in air to 1100°C for
24h. The
room temperature total hemispherical emissivity of the coating, as measured
from 2 - 20 Win, is 0.902. As in Example 17, protection of carbon is
demonstrated even at these elevated temperatures.
Example 19
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Solution mixture of Example 1 was modified via addition of an organic
component to enable development of thicker films that are crack-free. A piece
of 1018 carbon steel was coated with the aforementioned solution that yielded
a
relatively thicker coating (> 1 micron average) which was substantially crack-
free. The amount of organic additive can be varied to obtain a range of
thicknesses for the film deposited. The coated 1018 coupon was subjected to
salt fog chamber, per conditions specified in ASTM B 117 test along with an
uncoated coupon for four days. The coated coupon was substantially
corrosion-free while the coated coupon was almost fully corroded. This
demonstrates the substantially crack and pin-hole free nature of thicker
coatings of the inventive material.
Example 20
Using the process described in Example 19, a completely inorganic
coating that is substantially opaque and dark in appearance can be developed
on metallic and other substrates. A piece of stainless steel was coated to
yield a
black coating. The coating is substantially crack or pore-free, hard and
abrasion resistant and may have excellent weathering resistance as compared to
conventional black paints. Excellent adhesion is also promoted as described in
the specifications of the patent application.
Example 21
An alcoholic solution containing silver ions was added to the solution
mixture from Example 1. This solution was used to coat a soda-lime glass
slide. The applied coating was dried in flowing air to remove the solvent and
cured to form a substantially transparent inorganic coating.
Example 22
A slurry coating as described in Example 16 was applied to a portion of
a stainless steel coupon. The coated sample was immersed in pure molten
aluminum at 760°C and retracted. No noticeable wetting of aluminum was
observed on the coated portion of the coupon as compared to complete
coverage of the uncoated portion immersed in molten aluminum, thus
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demonstrating the chemical stability and compatibility with molten aluminum
and excellent non-wetting characteristics. (See Fig 20.)
Exam 1p a 23
A piece of 1018 carbon steel is dip-coated using the solution mixture of
Example 1. This coating is dried in flowing air until the solvent has
evaporated
and then heat treated at 500°C for 30 minutes in air. X-ray diffraction
of the
surface of the sample showed that an oxide scale of predominately Fe203 had
grown on the surface, with small amounts of Fe304. An uncoated piece of
1018 steel was also heat treated at 500°C for 30 minutes. X-ray
diffraction of
the surface showed predominately Fe304 formation, with a very slight amount
of Fea03 (Figure 21). Fe~03 is a substantially more protective oxide scale
than
Fe304. This also demonstrates the ability of the film of the inventive
material
to substantially alter the chemistry of oxide scale growth of metallic
substrates.
Example24
A piece of stainless steel is partially dip-coated in solution mixture of
Example 1 and cured above 500°C for few minutes in air.. Raman
spectra of
both the coated and uncoated portions of the coupon were taken. In the
spectrum of the coated part of the sample, three peaks are visible which do
not
correspond to the uncoated sample or crystalline aluminum phosphate or
amorphous aluminum phosphate obtained using methods prescribed in this
invention, thus suggesting that the peaks correspond to material formed
("interface layer") near the interface between the coating and the substrate.
Although the exact chemistry or nature of the interface layer is not known, it
is
believed that this helps in improving the adhesion of the coating. The nature
of
the interface layer may vary depending on the chemistry of the substrate
composition and the heat treatment conducted during and after the deposition
of the coating(Figure 22).
Example 25
A piece of stainless steel is dip-coated using the solution mixture of
Example l and cured using a procedure prescribed in Example 24. This coated
specimen is exposed to the ambient for a period of days. FTIR spectra
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collected from the surface shows the presence of organic species w (Figure
23).
The bonding of the organic appears to be robust and helps in further lowering
the surface energy and yields a substantially more non-wetting surface.
Example 26
The sample of Example 20 is immersed and retracted from molten
aluminum at 760°C. Significant portion of the coated portion appeared
to be
completely non-wetting to molten aluminum.
Example 27
A piece of steel was partially coated with the solution mixture of
Example 1. The coupon was dried with flowing air and heated at
500°C for a
sufficient time to cure the film (remove all of the organics and nitrates) to
form
substantially inorganic material. The surface energy of the coated sample, as
well as the uncoated, heat treated sample was measured. The surface energy of
the coated material was 31.89 mJ/ma.
so
